Encapsulation of substrates containing IC chips and other electronic circuitries is important for providing environmental protection and heat dissipation for the circuits on the substrates. Different methods of encapsulation are provided for the circuits on the substrates, depending on the types of substrates used and the environmental conditions the electronic circuits on the substrate are to operate under.
There are in general two types of encapsulation methods, namely the open-form encapsulation and the chamber encapsulation.
An example of the open-form encapsulation is the so called “dam and fill” method. According to this method, a damming epoxy of very low viscosity is formed around circuits on the substrate. A potting epoxy of a higher viscosity is dispensed into the region surrounded by the damming epoxy to flood the entire substrate. After the potting epoxy is hardened, the damming epoxy and the region outside are sawn off, leaving the encapsulated substrate. This method is slow and the costs of encapsulation per substrate unit are very high. There is also high wastage of material due to the sawing off of the damming epoxy. Further, this method has very poor encapsulating material thickness control.
An improvement to the “dam and fill” method is the screen-printing method. According to the screen-printing method, a stencil is used to cover the areas of the substrate that are not supposed to be encapsulated, and a screen print machine coats a layer of encapsulating material over the entire surface of the substrate. The printing process continues until the desired thickness of the encapsulation layer is reached. This method is faster and has lower substrate unit costs compared to the “dam and fill” method. However, the sweeping motion of the encapsulating material due to the sweeping action of the printer can cause disturbed wire problems of the circuits on the substrate. Furthermore, this method is limited to encapsulating one or two pieces of substrates at one time.
Transfer molding is a form of chamber encapsulation which provides excellent thickness control. Using transfer molding, multiple pieces of substrates can be encapsulated at one time and the substrate unit costs of encapsulating a substrate unit are low. This process is also very fast compared to the methods described above. According to this method, generally a top mold 101 with a rectangular cavity 104 is placed onto a substrate 100 which is supported by a bottom mold 102 under the substrate 100 as shown in FIG. 1. Clamps 103 are used to hold the top mold 101, the substrate 100 and the bottom mold 102 together. Encapsulating material is injected through openings 105 arranged in the top mold 101 into the cavity 104 enclosed by the top mold 101 and the substrate 100. The clamps 103 and the top mold 101 are subsequently removed and the encapsulating material on the substrate is cured so that it is hardened to form the desired encapsulation.
It is advantageous to use ceramic substrates since they offer smaller line and space features of the electronic circuitries, and hence smaller form factor packages especially for mobile applications. They also provide better thermal conductivity compared to organic substrates.
However, ceramic substrates are very brittle and would thus break very easily due to the application of the clamping force during the process of encapsulating the substrate using the transfer molding technique.
Therefore, nowadays, ceramic substrates are encapsulated only by using the open-form method, in particular using the “dam and fill” method.